Ardour, as with it Mixbus using GTKmm, as widget library to make some money. Therefore they distribute binary's in the manner like windows providers does.
or OSX or iOS etc or actually many other Linux vendors.
The motivation is to have specific hand-picked version of libraries. The vast majority of Ardour bugs was a result of distros mixing various libraries without understanding the relationships, in particular for realtime applications. You will find that projects which do have a similar long list of dependencies (Firefox, Blender etc) do the same. It's the only sane approach for a complex application that needs to be reliable.
Now, today, commercial DAW developers expect from plugin developers that they has to develop there own widget library. This is, in my view totally stupid. In my view, the other way round would make sense, if needed.
Which gtk widgets do you actually use in guitarix? They're all custom widgets are they not?
The actually stupid part is that from a FLOSS perspective there is no reasons for plugins at all. The source is available and could be directly built into the workstation. The commercial idea behind plugins is actually that you can mix various proprietary projects without sharing source.
The whole concept to load a couple of random binary blobs (even if you have the source) into the memory space of another application and expect them to do realtime processing and safely interact with a GUI and with each other is kinda crazy. It's a proprietary model, with FLOSS it would not be needed.
And yes it's rather demanding on the developer's side: If you want to create plugins you do need to play by the rules. You can't mix ABIs, nor make any assumptions on 3rd party libs. Developing some widgets and event-system is trivial compared to the details otherwise involved.
They expect from plugin developers to "not use what we use because we use it" .
It's actually plugin-devs who out of respect for other plugin devs don't do that. The host is a single entity with known properties to target, but plugins don't and cannot know about each other, so they must be self-contained.
As for toolkits suitable for audio plugins: The only one that does get most of the detailed issues right is JUCE (and some derivatives like DPF) -- It's also the only one that does have actual useful widgets for plugin-UIs.
Ardour, for example use for the binary's they provide, old, outdated library's which are normally part of your system,
They may be old, but they're hand-picked. And no, most of them won't normally be part of a default installed system.
I know, this isn't what you all wont to hear here, but, we are on the way to lose the control over our so beloved open source model of audio engineering.
Why do you think that, can you elaborate?
Large parts of this is actually because of the GPL and it's all free software. I fail to see the correlation that you imply.
Unless you didn't mean the development process but integration. But a plugin is never in control and never has been. Isn't that the whole point of a plugin (vs standalone app)?
I'm, as a plugin and host developer, do my best to provide compatible "binary's" for the majority of users, but, to be honest, that isn't possible for arch linux.
It definitely is possible, just look at all the JUCE plugins. Also various standalone ones, helm, zyn-fusion.. Just don't make any assumptions about the system and it'll also run on *BSD, Windows, OSX and every Linux system out there.
If you do depend on 3rd party libs and dynamically link against them or make assumptions about API/ABI, then yes, you can only target a specific system.